Microphones have been used in a variety of equipment such as mobile phones and IC recorders. An acoustic sensor built in each of such microphones is required to have an improved S/N ratio and a reduced size.
As a method for increasing an S/N ratio of the acoustic sensor, first, there is a method of increasing sensitivity of the acoustic sensor. In order to increase the sensitivity of the acoustic sensor of the capacitance type, there can be adopted a method of widening an area of a diaphragm and a method of reducing spring properties of the diaphragm to increase a displacement amount of the diaphragm. However, in the former method of widening the area of the diaphragm, reduction in size of the acoustic sensor is hindered. Further, in such a method of decreasing the spring properties of the diaphragm as the latter method, since the displacement amount of the diaphragm increases, the durability of the acoustic sensor decreases.
A second method for increasing the S/N ratio of the acoustic sensor is to reduce noise of the acoustic sensor. As the noise of the acoustic sensor of the capacitance type, thermal noise generated in an air gap formed between the diaphragm (movable electrode plate) and a back plate (fixed electrode plate) is problematical.
The thermal noise in the air gap is noise generated by a mechanism as shown in FIG. 1(A). As shown in FIG. 1(A), air molecules α present inside an air gap 13 between a diaphragm 11 and a back plate 12, namely a semi-enclosed space, are collided with the diaphragm 11 due to fluctuations (thermal motion). Microforce due to the collision with the air molecules α is applied to the diaphragm 11, and the microforce applied to the diaphragm 11 fluctuates at random. Therefore, the diaphragm 11 vibrates due to the collision with the air molecules α, to generate electric noise in a vibration sensor. Especially in a highly sensitive acoustic sensor or microphone, noise attributed to such thermal noise is large, and the S/N ratio thus deteriorates.
The noise attributed to such thermal noise can be alleviated by increasing an opening ratio of an acoustic hole 14 opened in the back plate 12 as shown in FIG. 1(B), to facilitate passage of air inside the air gap 13 through the acoustic hole 14. Further, the noise can also be alleviated by widening the air gap 13 between the diaphragm 11 and the back plate 12. However, when the opening ratio of the acoustic hole 14 is increased or the air gap 13 is widened, a capacitance of a capacitor configured of the diaphragm 11 and the back plate 12 decreases. For this reason, with the method of simply reducing noise, the sensitivity of the acoustic sensor decreases simultaneously with reduction in noise, and hence it has not been possible to improve the S/N ratio of the acoustic sensor.
(Conventionally Known Vibration Sensor)
Patent Document 1 discloses a microphone of a difference sensing system aimed at improving the S/N ratio. As shown in FIG. 2, in this microphone 21, two acoustic sensors 23a, 23b are provided on one substrate 22, and vertical conforms of both sensors 23a, 23b are inverted to each other. That is, in one acoustic sensor 23a, a fixed plate 25a having acoustic holes 26a is formed above a diaphragm 24a, to constitute a capacitor for acoustic sensing. In the other acoustic sensor 23b, a diaphragm 24b is formed above a fixed plate 25b having acoustic holes 26b, to constitute a capacitor for acoustic sensing.
With sensing signals outputted from the diaphragms 24a, 24b in the acoustic sensors 23a, 23b, when both acoustic sensors 23a, 23b detect the same acoustic vibration, sensing signals with phases displaced 180° are outputted from both sensors 23a, 23b. The output of the acoustic sensor 23a and the output of the acoustic sensor 23b are inputted into a signal processing circuit (ASIC), and subjected to subtraction processing inside the signal processing circuit. This results in adding up of the acoustic detection signals detected by both sensors 23a, 23b, whereby the detection sensitivity of the microphone 21 improves, and the S/N ratio is expected to improve.
In such a microphone of the difference sensing system, the detection sensitivity thereof decreases unless phases, frequencies and sensitivities of acoustic detection signals detected by the two acoustic sensors are completely the same. However, making characteristics of the acoustic sensors, separately formed on the same substrate, uniform is difficult. Further, when polarities of the capacitors in both sensors 23a, 23b are opposite to each other as in this microphone, producing two equivalent acoustic sensors 23a, 23b is difficult due to a parasitic capacitance. It has thus been difficult in practice to improve the S/N ratio in such a microphone as in Patent Document 2.
Further, in such a microphone, noise derived from mismatching tends to be generated, and hence there are limitations on improvement in S/N ratio.
Moreover, an extra computing function needs to be added to the signal processing circuit, which results in high cost of the signal processing circuit. There has also been a problem in that reduction in size of the microphone is difficult because of the need to provide a plurality of acoustic sensors on the substrate.
(Another Conventionally Known Vibration Sensor)
Patent Document 2 discloses another conventional microphone. This microphone 31 basically has a similar structure to that of the microphone 21 of Patent Document 1. As shown in FIG. 3(A), in the microphone 31 of Patent Document 2, a plurality of independent acoustic sensors 33a, 33b, . . . having the same structure are provided on a common substrate 32. That is, any of the acoustic sensors 33a, 33b, . . . is formed with a diaphragm 34 as opposed to the top surface of a fixed plate 35 in which acoustic holes 36 are opened. Further, as shown in FIG. 3(B), a signal processing circuit 37 is provided on the top surface of the substrate 32, and an output of each of the acoustic sensors 33a, 33b, . . . is connected to the signal processing circuit 37 through an electrode leader 38 wired on the substrate 32. In the case of this microphone 31, with each of the acoustic sensors 33a, 33b, . . . having the same structure, the output of each of the acoustic sensors 33a, 33b, . . . is subjected to addition processing in the signal processing circuit 37 so that the improvement in S/N ratio is expected.
However, even the microphone described in Patent Document 2 has a problem as follows. Since warpage that occurs in the diaphragm in the microphone producing process varies, variations in sensitivity among each acoustic sensor tend to be large. On the other hand, when the variations are intended to be made smaller, the productivity of the microphone decreases. Further, there has been a problem in that, when the electrode leader that connects between each of the acoustic sensors and the signal processing circuit on the substrate becomes longer, a parasitic capacitance and parasitic resistance of the microphone become larger, to cause deterioration in characteristics such as the sensitivity.
Moreover, since the plurality of independent acoustic sensors are provided, disagreement of the acoustic characteristics other than the sensitivity tend to occur among each sensor. For example, since the frequency characteristics, phases and the like are influenced by a back chamber and a vent hole of each sensor, each sensor tends to have different characteristics.
In the microphone of Patent Document 2, variations in sensitivity and other acoustic characteristics in each acoustic sensor tends to occur as thus described, and it has thus been difficult in practice to obtain the effect of improvement in S/N ratio.
Further, since the plurality of independent acoustic sensors needs to be arranged in array on the substrate, there has been a problem of the microphone being not reducible in size.